Various embodiments relate to a control method for semiconductor light sources which is suitable for applications that presuppose a rapid driving of the semiconductor light sources. This is the case in front and rear projection applications for example.
Recently, powerful semiconductor light sources such as high-power light emitting diodes have increasingly been used in applications which had previously been reserved for high-pressure discharge lamps. Precisely in the field of projection, the semiconductor light sources are not driven continuously but rather are operated in pulsed fashion in order to meet the requirements in this field. Very short pulses are employed, which in return have a very high power density.
Since, in present-day semiconductor light sources, the light emission is dependent to a greater or lesser extent on the temperature of semiconductor light sources themselves, a control is necessary which ensures that the quantity of light emitted by a projection unit remains constant. Since pulsed driving methods have only been employed for a short while, the control methods usually used only take account of heating of the semiconductor light sources in relatively large time spans, that is to say over a relatively long period of consideration. As an example, one conventional method can be mentioned in which the temperature of the heat sink or heat sinks connected to the semiconductor light sources is measured, and the driving of the semiconductor light sources is adapted in accordance with the heat sink temperature. This takes account only of long-term effects, but not of the immediate heating of the semiconductor light sources that takes place during driving with short, powerful pulses. Since the pulse pauses can occasionally be very long, the semiconductor light source has enough time to cool down before the next pulse, with the result that the average loading over time does not turn out to be excessively high.
However, the semiconductor light source heats up during such a pulse to such a great extent that the emitted quantity of light does not remain constant during the pulse, but rather decreases continuously. In display applications, this can lead to impairment of the backlighting quality, and hence of the picture quality, and is therefore undesirable.